Shower mixer tap only producing lukewarm water.

[bullethead123]

Hi,
I'm only getting lukewarm water from my shower mixer tap even when the temp knob is turned to the max. The water from the basin tap is fine.
Here's a couple of photos:

The top valve turns the shower on and the bottom valve controls the temp. Does this look like a thermostatically controlled device? I'm guessing not. What could be the problem? Some kind of blockage?
Thanks for reading.

 

[Harry Bloomfield]

Is the source of your hot water, a combi boiler, or an open vented system, with a cylinder and header tank perhaps in the loft?

 

[bullethead123]

Hi Harry, it's a combi. A viessman model.

 

[Harry Bloomfield]

Hi Harry, it's a combi. A viessman model.

Either you have a restriction in the hot supply, or if it is a thermostatic mixer - the thermostatic cartridge has failed. Turn the water supply off, and unscrew the unit, out of it's socket, in the wall, then post photos, to progress it further.

 

[bullethead123]

I removed the mixer valve and found an o ring unseated. It had migrated half an inch up the body of the valve. I replaced it for a new one (47p) and now the shower works fine.
Thanks for your help Harry.